It kind of looked like the fan was looking down trying to scoop the ball up and didn't see Sheffield coming.
All Fans should learn not to lean over the wall and interfere with the ball during play.
the guy was trying to pull a fast one, he was looking out into the field...nowhere near the ball....and his fist was in a swift uppercut action. It was clearly intentional....not likely to do serious harm....but definitely to mess witht he 'evil' yankee player and make contact with Sheffield's head.